High current drivers often consume a substantial portion of the total current required for integrated circuits. Emitter follower configurations are attractive as high current drivers because of their high input impedance, low output impedance, and high current sourcing capabilities. However, traditional differential NPN emitter follower circuits generally provide poor or inefficient mechanisms for current sinking and also no voltage gain.
FIG. 1 illustrates a prior art differential current amplifier using first and second transistors 170, 180, respectively, configured as emitter followers. The IN signal 110 forms one-half of a differential signal and is coupled to the base of the second transistor 180. The INX signal 120 forms the other half of the differential signal and is coupled to the base of the first transistor 170. For the purposes of this discussion, a differential signal is composed of two signals which are 180 degrees out of phase. The output signals OUT 130 and OUTX 140 are current amplified versions of the signals IN 110 and INX 120. Since the first transistor 170 and the second transistor 180 can only source current and have no means to sink current, a first current source 150 has been coupled to the emitter of the first transistor 170 and a second current source 160 has been coupled to the emitter of the second transistor 180. The amount of current which the first and second current sources 150, 160 are capable of sinking must be large enough to discharge any capacitance, presented at the output signals OUT 130 and OUTX 140, in a time period sufficient to meet the bandwidth requirements of the application. Since the first and second current sources 150, 160 constantly sink current regardless of the signal amplitude, they result in a waste of current. Further, when the first and second transistors 170, 180 source current in an attempt to charge any load capacitances, some of the current sourced is diverted to the first and second current sources 150, 160.
A push-pull amplifier is an alternative high current output configuration. However, push-pull amplifiers require the use of high quality PNP devices which are often not available in integrated circuits, and therefore, useable push-pull amplifier structures are often not realizable, especially where the amplifier is to be used at radio frequencies.
Accordingly, there is a need for a differential current amplifier circuit which does not require both PNP and NPN transistors, and which is more efficient than conventional emitter follower circuits.